Positive ion trap gun



Sept 8, 1959 D. D. VAN oRMER POSITIVE ION TRAP GUN Filed Sept. 16, 1954rally IN VEN TOR. /l V/a. MM 06M?? @l lva POSITIVE ION VATRAP GUN David'D. Vanrmer, Lancaster,`Pa.,"assignor to'Radio This-invention isdirected to a cathode ray tube and iparticularly to an electron gunstructure providing a positive ion trap for use in cathode ray tubessuch as television picture tubes.

Normally, a cathode ray tube for television or other ypurposes is onehaving 'an envelope with a large bulb portion and a tubular neck portionxed thereto.

Mounted Within the tubular neck portion are a plurality of electrodesfor forming and focusing an electron beam along a path extending intothe bulbportion. Magnetic or electrostatic field producing means 'areused forfmovingthe electron beamin any desiredlmanner over a wallportion -or face plate of theenvelope bulb portion. Theinner surface ofthe wall or face plate is coated With a iilm of phosphor material whichluminesces With a visible light when struck by the electron beam. Bymodulating the current of the electron beam the scanned area of thephosphor screen can be Varied in a manner to produce a light path inaccordance with modulating signals applied to the electron gun of thetube.

y Tubes of this type normally are highly evacuated. However, it has beenfound to be impossible to remove every trace of4 gaseous material fromthe tube envelope when it is sealed. Also, during the normal operatinglife of the cathode ray tube, certain occluded gases in the envelopewall or in other portions of the Ytube structure are caused to bereleased into lthe discharge space of the tube. The electron beam isnormally accelerated at high energies between thevcathode gun andthephosphor screen of the tube. The electron energies are sutlicient toionize gas particles which pass into the beam path during tube operationby knocking electrons out of the gas molecules forming positivelycharged -gas ions. The accelerating fields within the gun structure ofthe tube act on the positively charged ion particles and urge themsubstantially along the electron beam path toward the negative end ofthe electron gun. The positive ions being accelerated to high energieswill strike the cathode surface with sufficient energy to destroy theelectron emitting properties of the cathode material on the electrode.Positive ion bombardment of the cathode surfaces in tubes of this typecauses an early breakdown of the thermionic emission from the cathodesurface. This deleterious effect on the cathode electrode greatlyshortens the life of the cathode ray tube.

It is therefore an object of this invention to provide an electron gunfor a cathode ray tube which includes means for preventing positive ionbombardment of the cathode surface.

It is another object of the invention to provide a novel electrongun'means which includes structure'for trapping .positive ions formedinthe electron beam path.

It is a further object of this invention to provide an electron gun fortelevision Vpicture tubes which includes -means for preventing positiveions in the beam path from striking the cathode.

y The `invention is va novel electron -gun structure in 'which thebeam-forming electrodes of the gun are offset United States Patent O iceseffgsg from the normal axis to the phosphor screen surface or are otsetfrom the undeected beam path directed at the phosphor screen. Thebeam-forming electrodes of the gun establish an electron ybeam which isaccelerated toward the screen axis into a non-symmetric electron lenswhich bends the beam to align it W-ith the screen axis normal to thescreen at its center. The positive ions which in the absence of theinvention would bombard the cathode surface are only those which areformed in the beam path outside of the field-free regions of the tube.These positive ions will normally be different in energy than the energyof the electrons and the negative ions of the beam and upon passing backdown the beam path toward the cathode electrode, these positive ionslbecause of their difference in energy will be bent out of the beampathvby the electrostatic bending field and will be collected byportions of the beam-forming electrodes of the gun.

Figure l'is a sectional view of the cathode ray tube having a gunstructure in accordance with the invention.

Figure 2 is a schematic representation of the beam path of the gun ofFig. l.

Figure 3 is a cross-sectional View of another form of the invention.

The cathode ray tube envelop of Fig. 1 comprises` a large bulb portion10 and a tubular neck portion 12 which is substantially a cylinder ofglass formed withits axis 1'4 coincident with the axis of the bulbportion 10. The end of the bulb portion 10 opposite to the tubular neck12 consists of a glass Wall portion or face plate `16 upon which isformed a lm or coating 18 of finely rdivided phosphor particles, whichforms a phosphor screen.

Mounted Within the tubular envelope portion 12 is an electron'gun 20consisting of a cathode electrode22 mounted Within an enclosing tubularcontrol grid electrode 24. Closely spaced from the control grid 24-andaxially aligned therewith is a short cup-shaped tubular acceleratingelectrode 26. The electrodes 22, 24 and 26 are aligned on a common axiswhich makes a small angle with the axis 14 of the tubular neck 12.

Mounted` coaxially Within the tubular neck 12 is a second acceleratingelectrode 28 consisting of a tubular metal member closed at one end by acircular plate structure 30 having a centrally located aperture 32therein. Electrodes 24 and 26 are closed by centrally apertued platestructures 34 and 36 respectively. The apertures of these plates 34 and36 are aligned and are located on a common axis of the electrodes 24 and26. The adjacent ends of the electrodes 26 and 28 are cutjat an angleboth to the axis i electrode 28 and the common axis of electrodes 22,24,and 26. The centrally located aperture of plate 34, of control grid 2.4overlies one end ofthe cathode 22 which is closed by a solid Wallportion, the outer surface of which is coated with an electron emittingmaterial.

The apertured plate 30 of the electrode 2S is connected by springfingers 38 to a conductive coating {4i} formed on lthe inner surface ofthe tubular neck 12 and which extends from the neck 12 into the bulbportion 10 to a point adjacent the 'fluorescent screen 18. Theelectrodes of the electron gun 2l) are mounted in their spaced positionand are insulated from each other by ceramic vor'vg'lass support rods,which 'are not shown.

Operative potentials are applied to the several electrodes shown, byconnecting these Velectrodes with conductive leads '42, sealed through aglass Wall portion 44 closing the end of the 'tubular neck 12. Leads 42are coated cathode surface. Voltages are applied to the sevleralelectrodes having values of the order of those indicated in Fig. l.These potential values are not limiting but are illustrative of voltageswhich have been used in tubes of the type shown in Fig. l and for thepurposes to be described.

In operation, electrons emitted from the coated cathode surface areurged through the apertured grid plate 34- by the accelerating field ofelectrode 26 and are formed into an electron beam 50 directed along thecommon axis of electrodes 24 and 26. Potentials on electrodes 26 and 28form an electron lens field therebetween. Because the adjacent ends ofelectrodes 26 and 28 lie in parallel planes tilted at an angle to thecommon axis of electrodes 24 and 26 and also at an angle to the axis 14of electrode 28, the lens field formed between electrodes 26 and 28 isnon-symmetric with respect to these two axes. Therefore, the electronbeam 58 passing along the axis of electrodes 24 and 26 passes into thislens iield at an angle thereto and is bent toward the axis of the lensield which may be considered substantially normal to the parallel planesin which lie the adjacent ends of electrodes 26 and 28. The potentialdifference between electrodes 26 and 28 can be adjusted to control theamount of deflection or bending which the non-symmetric lens eld givesto electron beam 50. During tube operation the electron beam is bent toalign the beam on axis 14 and directed substantially normal to thefluorescent screen 18 at its center. The angle which the adjacent endsof electrodes 26 and 28 make with axis 14 may also be varied to providethe desired amount of beam bending for the voltage ranges used onelectrodes 26 and 28.

ln a successfully operated tube of the type described, the adjacent endsof electrodes 26 and 28 are formed at an angle `of substantially tendegrees to axis 14. The electron beam in passing through thenon-symmetric lens between electrodes 26 and 28 is given its iinal andgreatest acceleration so that in passing down the axis 14 of electrode28 the electron beam is moving with high energy into a field-free spacewhich extends from a point within tubular electrode 28 to the screen,since electrode 28 and the conductive coating 46 are maintained at acommon potential.

The electron beam 58 passing into the non-symmetrical eld betweenelectrodes 26 and 28 is a divergent beam. The non-symmetrical eld,provides only a small convergence to the beam so that the beam strikingapertured plate 30 is still somewhat divergent. A magnetic eld providedby a coil winding 52 is used to focus the electron beam to a small spoton fluorescent screen 16. Coil 52 is enclosed in an iron armature casing54 to provide a concentrated focusing eld on the tube axis 14. Theelectron beam 50 is scanned over the surface of target screen 18 bydeflecting magnetic iields formed by pairs of deiiecting coils mountedin a neck yoke 56 on the tubular neck portion 12. These deecting coilsconsist of pairs of coils connected in series to sources of sawtoothcurrents for providing respectively line and frame scansion of theelectron beam. Both the focusing means 52 and the deecting means 56 areconventional and do not constitute a part of this invention.Accordingly, they are not described in greater detail.

The scansion of beam 50 of screen 18 may be in any well known mannersuch as to provide for example a rectangular raster normally used intelevision picture tubes. The intensity of electron beam 50 is normallyvaried in accordance with signal voltages applied to the control grid24. Thus the beam 58 can be modulated in accordance with video signalsapplied to screen 24 to provide a charge pattern of light on theuorescent screen 18, such as a television picture.

In tubes of the type described and shown in Fig. l, it is desirable thatthe tube envelope be completely evacuated for optimum operation. Tracesof gas within the tube envelope provide a source of positive ions whenstruck by the high energy electrons and negative ions of beam 58. Sincethese ions are positively charged they pass down the electron beam path58 and are accelerated by the electrostatic ields along the path tostrike the cathode electron emitting surface at high energies. Positiveion bombardment of the cathode surface causes a deterioration of theemitting characteristics of the coating and tend to greatly shorten thelife of the discharge tube.

Therefore, in accordance with the invention, the cathode 22 is oisetfrom the tube axis 14 and a non-symmetric electrostatic lens is formedalong the beam path between the cathode 22 and screen 18 and at theportion of the beam path at which the electron beam has its greatestenergy.

Fig. 2 is an enlarged schematic representation of the electron beam path50 extending from cathode 22 into the tubular electrode 28. Indescribing the invention it is only necessary to consider the effect ofthose positive ions formed along the electron beam path by the directcollision of electrons and negative ions of the electron beam with gasmolecules present in the beam path, since it is only those positive ionsformed by direct collision which will pass back down the beam path tothe cathode. Positive ions formed by other than direct collision withelectrons or negative ions will be deflected away from the beam path andwill be collected by electrode structures of the gun 20.

A direct collision between a gas molecule and a negative beam particlewill give the positive ion formed an initial motion along the beam pathin the direction of the screen 18. Considering the positive ions formedWithin the electrode cylinder 28 between the bending lens ield andscreen 18, if an ion is formed by direct collision within electrode 28,at point A (Fig. 2) it may receive suicient energy to force the positiveion toward screen 18 and into the field-free space extending from withinelectrode 28 to screen 18. This positive ion is thus carried out of anyaccelerating eld and will not be drawn back along the beam path 50 tothe cathode 22.

A positive ion formed by direct collision with a negative particle atpoint B Within electrode 28 will be first forced by the action of thecollision along the beam path toward screen 18, but because of theaccelerating negative fields between point B and the cathode 22, theenergy of collision will be insuflicient to carry the positive ion intothe ield free space, but instead will cause the ion to reverse thedirection of its motion and to be accelerated back along the beam path50. As the positive ion passes the point of collision, B, it will havegained the energy of formation and will be further accelerated to enterthe deecting field between electrodes 26 and 28 at a higher energy thanthe energy of the electron beam passing through the deiiecting field inthe opposite direction. That is, the positive ion will enter thedeflecting ield with substantially all of the energy imparted to it atcollision in addition to the energy given to the positive ion by thehigh accelerating field between electrodes 26 and 28. Since thedeflection of a moving charged particle by an electric field isinversely proportional to the kinetic energy of that particle, positiveions passing into the deflecting field between electrodes 26 and 28 willnot follow the electron path to the cathode surface, since due to theirinherently greater energy they will be deiiected less than the electronsof the beam following the path 50. Thus, these high energy positive ionswill not pass through the aperture of plate 36 but will be collectedthereby and thus be prevented from striking the coated surface ofcathode 22.

Positive ions formed by direct collision in the deecting region, betweenelectrodes 26 and 28, at points C and D for example, and at other pointswith sufficient energy to carry them into this field, will be deected bythe non-symmetric field oppositely to the electron beam and will berandomly scattered. Positive ions, however, formed between the cathodeand the deecting field at E, for example, with insucient energy offormation to drive them to the deflecting field will be returned to thecathode. However, positive ions formed within this region (at E) areformed at low energies and since they are not accelerated to highenergies due to the low potential diierence between electrode 26 andcathode 22, the positive ions in this region will strike the cathodesurface at relatively low energy. To provide complete trapping of thepositive ions Within this region between the cathode and the deectingfield, it would be necessary to maintain the positive potential ofelectrode 26 below the ionizing energy of the gas molecules present, andthus prevent the negative ions or electrons of the beam, accelerated byelectrode 26 in this region, from having an ionizing effect on gasmolecules in the beam path.

An alternative structure Which would provide trapping of positive ionsin the low potential field adjacent to the cathode electrode isschematically shown in Figure 3. 'Ihis figure discloses a neck portion7i) of a cathode ray tube andv corresponding in structure to the neckportion 12 of Figure 1. Mounted within the neck 70, by any appropriatemeans (not shown), is an electron gun structure 71. The gun structure 71consists of a conventional cathode electrode 72 insulatingly mounted ona ceramic disc 74 Within a tubular control grid electrode 76. Controlgrid 76 includes an end wall 75 spaced from one end of cathode 72 whichis closed and coated with electron emitting material. Cathode 72 andcontrol grid 76 are coaxially arranged along a common axis '73 makingsubstantially a 45 angle with the axis 78 of the tubular neck portion70. An accelerating electrode 8G includes a plate portion 82 overlyingand spaced parallel to the end wall portion 75 of control electrode 76.Spaced from electrode 80 is a second accelerating tubular electrode 84coaxially mounted on the axis '78 of the tubular neck 70.

Control grid end wall 75 and accelerating electrode plate 82 each haveapertures therethrough substantially at their centers and overlying thecoated end of cathode 72. The apertures in plate portions 75 and `82,however, are ofset from each other or staggered, the aperture of plate75 being on axis 73 and the aperture in plate 82 being slightlydisplaced from axis 73 in a direction away from the axis 78. ElectrodeSti may have a portion 86 extending from plate 82 to a point spaced fromthe'adjacent end of tubular electrode 84, substantially as shown.

To operate the gun structure of Figure 3, appropriate voltages areapplied in the order of the values disclosed for correspondingelectrodes of Figure l. Cathode 72 is heated by a lament (not shown) toa thermionic temperature. Electrons, are emitted from the heated cathodeelectrode 72 and are eccelerated by electrode plate 32 through theoffset apertures in adjacent plates 75 and 82. Because of the offsetarrangement of these apertures, the electrostatic field betweenelectrode portions 75 and 82 form an asymmetric electron lens whichbends the electrons away from axis 73 and along a path which coincideswith axis 78, as the beam enters the accelerating electrode 84. Theelectron beam is thus directed down the tube axis 78 by the acceleratinglens field as described above for Fig. l. The beam then may be focusedand scanned over a fluorescent screen in the manner described above forthe structure of Fig. l.

The gun structure of Fig. 3 provides a bending field which extendsthrough the aperture of grid plate 75 into the region adjacent to thecathode electrode 72. This bending field includes portions of theaccelerating field of plate 82 which are below the ionization energiesof gases. The negative particles of the beam passing through thenegative grid aperture of plate 75 are eccelerated only by theasymmetric iield of the off-set apertures. Positive ions formed in theregion between plates 75 and 82 will thus be in an accelerating bending`field. These positive ions will normally have a diferent energy than thenegative particles in the electron beam so that when they are urgedtoward the cathode 72, they will be bent' differently by the asymmetriceld and will pass out' of the electron path to be captured by adjacentelectrode portions. in the bending field between plate 82 and electrode84,v will' also be deflected oli of the electron path as described'above for the structure of Fig. l. Thus, all points of the beam pathbetween cathode 72 and the final accelerating electrode 84, and wherethe negative particles of the beam reach ionizing energies, will be`within the beam bending field. The structure then of Fig. 3 discloses ameans for a complete trapping of all positive ions formedbetween thecathode and the phosphor screen of the cathode ray tube.

As pointedout above, the positive ions formed along, the electron beampath are trapped by the structure disclosed. The elimination of thesepositive ions eliminates the deleterious ion'bombardment of the cathodesurface, land thus results in a longer cathode life to extend theoperational life of the cathode ray tube. The trapping ofthe positiveions iseiectively done by a non-symmetric electrostatic beam bendingfield between the cathode electrode and the point of formation of thepositive ions.

What is claimed is:

1. An electron gun for forming a beam of negative particles along apath, said electron gun including an' accelerating electrode aligned onan axis along said beam path, a' cathode electrode spaced from saidaccelerating electrode and offset in one direction from said axis forprovidingV an electron emission, and tubular electrode means along saidbeam path between said cathode and accelerating electrode for providingan electrostatic eld lens for bending said beam in said one direction toalign said beam with said axis.

2. An electron gun for forming a beam of negative particles along apath, said electron gun including a first accelerating electrode alignedon an axis along said beam path, a cathode electrode spaced from saidaccelerating electrode and offsetV in one direction from said axis, anda tubular second accelerating electrode adjacent to said firstaccelerating electrode and between said first ac; celerating electrodeand said cathode electrode, said secondaccelerating electrode beingaxially aligned with said cathode Aelectrode and'oiset in said onedirection from said axis for providing an electrostatic field lens forbending said beam in said one direction to align said negative beamwithk said axis. v

3. An electron gun for forming a beam of negative particles along'apath, said electron gun including a first accelerating electrode alignedon an axis along said beam path, a cathode electrode spaced from saidaccelerating electrode and offset in one direction from said axis, and atubular second accelerating electrode mounted adjacent to said firstaccelerating electrode and between said first accelerating electrode andsaid cathode electrode, said tubular second accelerating electrode beingaxially aligned with said cathode electrode and offset in said onedirection from said axis, the adjacent ends of said first and secondaccelerating electrodes lying in parallel planes disposedl at an angleto said axis for bending said beam in said one direction to align saidbeam along said axis.

4. An electron gun for forming a beam of negative particles along apath, said electron gun including an accelerating electrode aligned onan axis along said beam path, a cathode electrode spaced from saidaccelerating electrode and offset in one direction from said axis, andelectrode means along said beam path between said cathode andaccelerating electrodes for providing an electrostatic field for bendingsaid beam in said one direction to align said negative beam with saidaxis, said electrode Also, positive ions formed means including a pairof parallel plates each having an aperture therethrough, said aperturesbeing oifset from each other relative to said axis.

5. An electron discharge device comprising an electron gun for forming abeam of negative particles along a path, said electron gun including anaccelerating electrode aligned on an axis along said beam path, acathode electrode spaced from said accelerating electrode and offset inone direction from said axis, and electrode means along said beam pathbetween said cathode and accelerating electrodes for providing anelectrostatic eld aligning said negative beam with said axis, saidelect-rode means including a pair of electrode members each having anaperture therethrough, said apertures being positioned in line with aportion of said cathode emitting surface and offset from each otherrelative to said axis for bending said negative beam in said onedirection to align said beam along said axis.

6. A cathode ray tube comprising an envelope having a tubular portion, afluorescent screen within said envelope, an electron gun structurespaced from said screen and including a cathode electrode within saidtubular envelope portion for producing a beam of electrons along a pathoffset in one direction from the axis of the tubular envelope portionwherein positively charged ions may be present in said electron beam,means between said cathode electrode and said screen for providing aneletrostatic field for bending said electron beam in said one directionto align said electron beam with said tubular envelope axis.

7. A cathode ray tube comprising an envelope having a tubular portion,an electron gun structure including a cathode electrode within saidtubular envelope portion for producing a beam of electrons along a pathoffset in one direction from the axis of the tubular envelope portionwherein positively charged ions may be present in said beam, saidelectron gun structure including as a part thereof means for forming anon-symmetric electrostatic field in the path of said beam for bendingsaid electron beam in said one direction to align said beam with saidtubular envelope axis.

8. A cathode ray tube comprising an envelope having a tubular portion,an electron gun structure including a cathode electrode within saidtubular envelope portion and oifset in one direction from the axisthereof for producing a beam of electrons along a path whereinpositively charged ions may be present in said beam, said electron gunstructure including as a part thereof means for forming a non-symmetricelectrostatic iield in the path of said beam for bending said beam insaid one direction to align said beam with said tubular envelope axis,said means including a tubular electrode coaxially mounted Within saidtubular envelope portion and a second electrode adjacent to said tubularelectrode and mounted between said tubular electrode and said cathodeelectrode, adjacent portions of said tubular and second electrodes lyingin parallel planes tilted in said one direction and at an angle to saidaxis, said electron gun structure including impermeable structureclosely spaced from said beam path for collecting positive ions directedtoward said cathode.

9. A cathode ray tube including an electron gun for forming a beam ofnegatively charged particles in which positive ions may be present, saidelectron gun including 'a cathode electrode having an electron emittingsurface and a pair of electrodes each having an aperture overlying saidcathode surface, an accelerating electrode having a centrally disposedaperture therethrough and mounted on an axis through said centrallydisposed aperture, said pair of electrodes being iixed so that astraight line from said cathode emitting surface passes through theapertures of said pair of electrodes at an angle to said axis, theapertures of said pair of electrodes being otset from each otherrelative to said straight line.

l0. The method of separating positive ions from the negative particlesof a cathode ray beam Within a cathode ray tube including a cathodeelectrode and a fluorescent screen, said method comprising the steps offorming the negative particle beam along a path offset from a secondpath directed at the phosphor screen, using an electrostatic lens fieldnon-symmetrically disposed with said beam path to align said negativeparticle beam with said second beam path, collecting the positive ionswhich are separated from the negative particle beam by thenonsymmetrical electrostatic lens before the positive ions strike thecathode.

l1. The method of separating positive ions from the negative particlesof a cathode ray beam within a cathode ray tube including a cathodeelectrode and a fluorescent screen, said method comprising the steps offorming electrous from said cathode into a beam which may contain othernegative particles and which is displaced from the axis substantiallynormal to said fluorescent screen at its center, directing said negativebeam along a path toward said axis through an electrostatic lens eldasymmetrieally disposed relative to said beam path and said axis,adjusting the strength of said asymmetric eld to align said negativebeam with said axis.

12. The method of separating positive ions from the negative particlesof a cathode ray beam within a cathode ray tube including a cathodeelectrode and a fluorescent screen, said method comprising the steps of,forming electrons from said cathode into a beam which may contain othernegative particles and which is displaced from the axis substantiallynormal to said fluorescent screen at its center, directing said negativebeam along a path toward said axis through an electrostatic lens fieldasymmetrically disposed relative to said beam path and said axis,adjusting the strength of said asymmetric iield to align said negativebeam with said axis, collecting positive ions separated by theasymmetric lens from the negative particle beam before the positive ionsstrike the cathode electrode.

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